This invention relates to fabrication of lithium-ion cells or batteries, and more particularly to improved activation methods therefor.
Batteries are widely used in the modern world for portable devices. Rechargeable batteries are convenient, in that there is a very limited disposal problem and the batteries need not be purchased and installed in the portable equipment after each discharge. It is always desirable to have higher energy density (higher energy per unit volume of battery) or higher specific energy (higher energy per unit mass). Ideally, a battery or its cells should be sealed to prevent leakage and avoid contamination of the cells.
At the current state of the art, lithium polymer cells have greater specific and volumetric energy, but are not yet commercially available, partially because of unresolved problems relating to the solid electrolyte. As a practical matter, lithium-ion liquid-electrolyte cells have the highest specific and volumetric energy density. Lithium-ion liquid-electrolyte cells are widely used in laptop computers and in cell phones.
Liquid-electrolyte lithium-ion cells in the prior art are made with graphite-and-binder coated copper-foil anodes and lithium cobalt oxide/binder/conductive-carbon-coated aluminum oil cathodes. The cell is filled with an electrolyte formulation which is generally an organic aprotic liquid solvent such as a formulation containing ethylene carbonate, dimethyl carbonate, and lithium hexaflurophosphate. After the cell is filled with electrolyte, it is xe2x80x9cactivated,xe2x80x9d which generally means preparing the surfaces of the anode and cathode for use, and leaving the cell in a substantially discharged state ready for initial charge by the user. It also prepares the surface of the anode with a solid electrolyte interface as a receptacle for Li ions.
The activation of a lithium ion liquid-electrolyte cell is described by Rosamaria Fong et al. in an article entitled xe2x80x9cStudies of Lithium Intercalation into Carbons using Nonaqueous Electrochemical Cellsxe2x80x9d, published at volume 137, page 2009 of Journal of the Electrochemical Society, 1990. As described by Fong et al., the cell is filled with electrolyte and sealed. Following the sealing, it is charged at a current of 0.14 mA/cm2 for 25 to 40 hours, followed by discharge of the cell at about 0.1 mA/cm2. The current density is based upon the mating surface area of the smaller one of the electrodes. It has been discovered that this protocol is disadvantageous, because the pressure within the sealed cell tends to rise during the charge, and may result in cell deformation and damage. In addition, it has been found that the Fong et al. charging procedure tends toward the longer of the stated times, namely 40 hours, and the discharge tends to take greater than five hours, for a total exceeding 45 hours.
A method for fabricating an activated lithium-ion cell according to an aspect of the invention includes the step of providing an anode comprising a carbonaceous insertion compound, which preferably should contain graphite. A cathode comprising lithiated metal oxide is provided. In a preferred embodiment, the lithiated metal oxide is manganese spinel. The anode and cathode are juxtaposed, separated by a dielectric sheet which is porous to ions and acts as a medium to transport lithium ions, to thereby define a cell. The cell is filled with an electrolyte in which lithium salt is dissolved to thereby produce an electrolyte-filled cell. Such electrolytes tend to decompose into their constituents at a given voltage. Following the step of filling the cell, the cell is charged at a first current density for a period in excess of one hour. In this context, current density is measured in amperes per unit area of the cell electrode. The preferred initial charging current density is about xc2xc to ⅓ mA/cm2, and the duration of such charging is preferably six hours. Following the step of charging the cell at a first current density, the cell is essentially open-circuited for a further period in excess of one hour, to allow the electrolyte to be distributed through the anode. The preferred duration of the open-circuiting is eight hours. Following the step of open-circuiting, the cell is charged at a second current density, which second current density is greater than the first current density, to a voltage less than the given voltage and greater than a predetermined voltage. In a preferred mode of the method, the second current density ranges from about twice the first current density to about 100 times the first current density. When the first current density is xc2xc mA/cm2, the second current density is preferably {fraction (1/2 )} mA/cm2. The charging at the second current density takes place until the cell voltage reaches the predetermined voltage, but not for so long a time that the cell voltage rises to the given voltage, so as to avoid decomposition of the electrolyte. The predetermined voltage is that voltage which provides the desired cell capacity. In the preferred mode, the cell is charged at the second rate until the cell reaches about 4.1 volts at which predetermined voltage the cell reaches about maximum capacity. During the charge at the second current density, the cell may produce some gas, which should be vented. Following the charging at the second current density, the cell is discharged at a third current density to a voltage which represents at least one lithium ion to sixty carbon atoms in the anode, which is about 2.5 volts. The third current density is at least ten times, and as much as a hundred times, the first current density. When the initial current density is xc2xc mA/cm2, the discharge current is preferably xc2xc centiampere/cm2. This discharge tends to reduce the energy in the cell, but leaves a sufficient density of lithium ions in the anode to support the spaced-apart plates of the graphite structure. The resulting activated cell can then be sealed into a protective metal sleeve, and is ready for charging by the user.
In a preferred mode of the invention, the step of filling the cell with electrolyte is accompanied by the step of allowing the cell to stand after the filling, and before the initial charging, for a period of at least one hour, and preferably twelve to sixteen hours, in an inert-gas, preferably argon, atmosphere.